1. Technical Field
This invention relates generally to the field of electrostatography, and more particularly to improvements in apparatus for detecting the concentration of toner particles in a development mixture of carrier and toner particles.
2. Background Art
In electrostatography, electrostatic images formed on a dielectric recording element are rendered visible via the application of pigmented, thermoplastic particles known as toner. Typically, such toner forms part of a two-component development mixture consisting of magnetically-attractive carrier particles and toner particles. The carrier and toner particles adhere together via triboelectric forces. During the development process, the electrostatic forces associated with the latent image act to strip the toner particles from their associated carrier particles, and the partially denuded carrier particles are returned to a reservoir.
It is well known in the art to continuously monitor the toner concentration in an electrostatographic development mixture and to replenish the mixture with toner when the concentration thereof falls below a predetermined level. For example, commonly assigned U.S. Pat. No. 4,141,645, which issued to M. G. Reid et al. on Feb. 27, 1979, includes a toner monitor which inspects the development mixture through an optical window in the reservoir to generate a signal related to the mixture's reflectivity; and therefore to its toner concentration. While such systems are generally effective, they are subject to some drawbacks. For example, the carrier and toner particles must be optically distinguishable (as in the case of a dry toner/carrier mixture) or the carrier particles must be transparent or semitransparent (as in the case of a liquid developer system). Optical window systems are also subject to problems of scumming and dust contamination.
Other prior art systems expose one or more test patches in non-image areas of the recording element. The measured optical density of the developed patches are good indications of the operation of the electrostatographic system, including the concentration of the development mixture. However, such systems require that a portion of the recording element be dedicated to the patches, either beyond the lateral edges of the image areas or in the interframe regions, thus making the equipment larger and the recording element more expensive. Toner particles are carried through the system but do not get transferred to receiver sheets. If improperly cleaned, the toner particles are a potential source of contamination of charges, optics, lamps, etc.
There is a suggestion in U.S. Pat. No. 4,200,665, which issued to K. Suzuki er al. on Apr. 29, 1980, at column 1, lines 56 to 66, that there are some prior art systems in which the toner is electrostatically attracted to the surface of an electrode plate to which a fixed voltage of the oppostite polarity to that of the toner is applied, or to the surface of a dielectric member which is electrically charged with a polarity opposite to that of the toner, followed by irradiation of the surface with light. The amount of light which has transmitted through or which is reflected from the surface is related to the amount of toner attracted to the electrode plate or dielectric member, and hence to the toner concentration in the developer mixture. If such a system does in fact exist in the prior art, it requires an electrical charging apparatus. As such, the system would require space for the charging apparatus, its cost would be increased by the cost of that apparatus, and would require additional electrical power to operate the charger. The additional power requirement would be an important consideration for battery-operated, portable copiers.